1. Field of the Invention
The present invention generally relates to a semiconductor optical waveguide structure for use in a semiconductor optical integrated circuit, and particularly to a semiconductor optical waveguide device that includes bent waveguides integrally formed in a single semiconductor substrate and provided with two or more reflection parts therein.
2. Description of the Related Art
In recent years, optical elements manufactured on a single semiconductor chip to form a monolithic integrated circuit structure are provided, so that a circuit device can be produced in small size, at low cost, with high reliability, and with increased productivity. In order to utilize enormous volumes of transmission data obtained by high-performance information and communication instruments, it is now indispensable to widely spread the use of optical communication networks with optical fibers. A variety of semiconductor optical devices can be constructed using a single substrate type of semiconductor. In the optical communication networks, therefore, semiconductor optical waveguides are widely used as optical transmission waveguides. Such semiconductor optical waveguides can be integrated to achieve a high speed operation with reduction in size. Hereinafter, an assembly of semiconductor optical waveguides having a structure of a combination of different types of semiconductor optical elements integrated in or on a single semiconductor chip substrate is referred to as a “semiconductor optical waveguide device” or simply as a “semiconductor optical device.”
In the semiconductor optical device, an optical waveguide having an S-shaped bent portion is generally used to change a light traveling direction as suggested, for example, in a non-patent literature 1 as below.
Non-Patent Literature 1: P. V. Studenkov and other six authors, “Monolithic Integration of an All-Optical Mach-Zehnder Demultiplexer Using an Asymmetric Twin-Waveguide Structure, “IEEE Photonics Technology Letters, Vol. 13, sixth edition, June, 2001, p.600, FIG. 1
FIG. 5A is a perspective view schematically showing a conventional semiconductor optical device with an S-shaped bent waveguide. FIG. 5B is a sectional view along line VB—VB of FIG. 5A. In FIGS. 5A and 5B, reference numeral 1 represents an optical waveguide, 11 an n-type InP substrate of a semiconductor chip, 12 an InGaAsP core layer of the optical waveguide, 13 a p-type InP clad layer, and 14 a semi-insulating InP buried layer. Incident light coming from an input end P travels through the S-shaped bent waveguide and shifts in a lateral direction by a distance X and then it goes out from an output end Q. In such a bent waveguide structure, it is important to reduce optical loss caused by bending. The bending loss is drastically increased, as the radius of curvature of the bent portion of the waveguide becomes smaller than a specified value.
FIG. 6 is a graph showing a relationship between the optical loss due to waveguide bending and the radius R of curvature of the bent portion. The relationship is calculated by the inventors according to a beam propagation method. The graph suggests that the radius of curvature should be 2400 μm or more for the reduction of the bending loss.
As another known application example of the conventional waveguide-type optical device with a high operation speed, small size and high efficiency, there has been suggested a waveguide type optical switch that includes multi-connected directional couplers, reflecting films each formed on a cleavage plane of one end of each directional coupler, and a plurality of optical waveguides formed by diffusing Ti substance in a form of stripes on a LiNbO3 substrate as disclosed, for example, in a patent literature 1: Japanese Patent Laid-Open H3-256028 (1991), FIGS. 1 and 2.
In the conventional structure as shown in FIGS. 5A and 5B, however, the length L between the input and output ends of the waveguide is relatively long. More specifically, in the case of the semiconductor optical waveguide and letting the radius of curvature be a minimum of 2400 μm, the length L between the input and output ends of the waveguide for shifting the traveling light in a lateral direction by 200 μm can be given by a geometrical calculation according to formula 1 as follows:L=2×(R−X/2(N+1))×tan(a cos((R−X)/2(N+1))/R))  (Formula 1)Substitution of X=200 μm, N=1 for the number of Y-branched part and R=2400 μm into formula 1 yields the length L=1371 μm, which is a relatively large size. This leads to a large size and a high cost of the semiconductor optical device. In order to reduce the device in size and cost, it is immediately necessary to reduce the length L between the input and output ends of the semiconductor optical waveguide while suppressing the bending loss.
On the contrary, in the structure as disclosed in the patent literature 1, the optical waveguides are formed by diffusing Ti substance in the form of stripes on the LiNbO3 substrate having a light refraction index of 2 or less (about 1.5), where the multi-connected directional couplers are provided and each of the reflecting films is formed on the cleavage plane of one end of each directional coupler to thereby achieve the relatively short length L of the device. In such a structure, however, neither the substrate nor the optical waveguide is made of a semiconductor material, and therefore, the device has some difficulty in having a high operation speed or high integration with a small size, and the substrate also has a large size due to its relatively low refractive index.